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Study suggests Roux-en-Y bypass leads to fat aversion

A new mechanism suggests gastric bypass surgery alters people’s food preferences
Carel le Roux

According to findings published in the American Journal of Physiology – Regulatory, Integrative, and Comparative Physiology (July 2011) gastric bypass surgery alters people’s food preferences so that they eat less high fat food. The study, which was led by researchers at Imperial College London, suggests a new mechanism by which this type of bariatric surgery can lead to long-term weight loss.


“It appears that after bypass surgery, patients become hungry for good food and avoid junk food not because they have to, but because they just don't like it any more,” said Dr Carel le Roux, Imperial Weight Centre at Imperial College London, UK, who led the research. “If we can find out why this happens, we might be able to help people to eat more healthily without much effort.”


This study investigated how gastric bypass affects intake of and preference for high fat food in an experimental (rats) study and within a trial setting (humans).


Rat experiments


In the rat experiments, rats given gastric bypass surgery were compared with rats that were given a sham operation. Rats that had gastric bypass surgery ate less food in total, but they specifically ate less high fat food and more low fat food. When given a choice between two bottles with different concentrations of fat emulsions, the rats that had gastric bypass surgery showed a lower preference for high fat concentrations compared with rats that had a sham operation.


The rat experiments suggested that the reduced preference for high fat food was partly due to the effects of digesting the food. There was no difference in preferences between gastric bypass rats and sham-operated rats when the rats were only given access to the bottles for a few seconds, suggesting that bypass rats did not dislike the taste of high fat emulsions when they were only allowed small volumes at a time.


Rats can learn to avoid foods that they associate with illness, so the researchers tested whether high fat foods would condition them to avoid certain tastes. They gave the rats saccharine-flavoured water while infusing corn oil into their stomachs. The gastric bypass rats learned to avoid saccharine, but the sham-operated rats did not, suggesting that the effect of digesting corn oil was unpleasant to the rats that had had gastric bypass surgery.


Levels of the satiety-promoting hormones GLP-1 and PYY were higher after feeding in the gastric bypass rats compared with sham-operated rats, suggesting a possible mechanism for the changes in food preferences. The team at Imperial plan to study the role of these hormones further to see if it might be possible to mimic the effects of gastric bypass without using surgery.


“These findings suggest that changes in fat preference may contribute to long-term maintained weight loss after gastric bypass” Carel le Roux

Human investigations


The researchers used data from 16 participants in a study in which obese people were randomly assigned either gastric bypass surgery or vertical-banded gastroplasty. At one and six years after surgery, these patients were asked to fill out questionnaires that included a series of questions to determine whether they avoided certain foods. The participants who had had gastric bypass had a significantly smaller proportion of fat in their diet six years after surgery, based on questionnaire responses.


Proportion of dietary fat in gastric bypass patients was significantly lower six years after surgery compared with patients after vertical-banded gastroplasty (p=0.046). Gastric bypass reduced total fat and caloric intake (p<0.001) and increased standard low fat chow consumption in comparison to sham controls (p<0.001) in rats. When compared to sham-operated rats, gastric bypass rats displayed much lower preferences for Intralipid concentrations above 0.5% in an ascending concentration series (0.005%, 0.01%, 0.05%, 0.1%, 0.5%, 1%, 5%) of two bottle preference tests (p=0.005). This effect was demonstrated 10 and 200 days after surgery. 


However, there was no difference in appetitive or consummatory behaviour in the brief access test between the two groups (p=0.71) using similar Intralipid concentrations (0.005% through 5%). Levels of GLP-1 were increased after gastric bypass as expected. An oral gavage of 1ml corn oil after saccharin ingestion in gastric bypass rats induced a conditioned taste aversion. These findings suggest that changes in fat preference may contribute to long-term maintained weight loss after gastric bypass. Post-ingestive effects of high fat nutrients resulting in conditioned taste aversion may partially explain this observation; the role of GLP-1 in mediating postprandial responses after gastric bypass requires further investigation.


Conclusion


The study outcomes indicate that Roux-en-Y gastric bypass patients decrease their consumption of both solid and liquid dietary fat. However, this does not appear to be triggered by taste, but rather influenced by the effects after ingestion. The hormone GLP-1 might be partly responsible for these effects.


“These findings suggest that changes in fat preference may contribute to long-term maintained weight loss after gastric bypass,” the authors note. “By elucidating the mechanisms by which obesity surgery reduces the consumption of high fat foods, new surgical and non-surgical therapies could be developed that mimic these mechanisms to offer safe and effective weight loss.”

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